This invention relates to a coil arrangement which permits imposition upon the magnetic field of a dipole arrangement a desired field distribution or compensation for an undesired field distribution. The coils are arranged between the pole shoes of a magnet.
In electron and ion optics, magnetic fields are employed which realize a predetermined field distribution over a large area (reported in: H. Wollnik; Nuclear Instr. and Meth., 95 (1971) 453). The required areas are in the order of sq.cm to sq.m, and the deviations from the desired field density distribution are in the order of magnitude 10.sup.-3 to 10.sup.-5 of the rated value. Up to now, the deviations of the real field from the nominal field have been compensated by floating pole shoes (Purcell filter, U.S. Pat. No. 2,962,636) or by the so-called shimming (precise removal of pole shoe material or applying of iron foils to the pole shoes).
The deviation from a homogeneous field distribution can be reduced by a Purcell filter only to the extent that it is caused by the magnet yoke. Also, the losses and further cost progressively rise with the area size.
With shimming, not only field distributions can be homogenized, but also a desired structure may be introduced into the field distribution. The process is quite complex, however, in particular, if material is to be removed from the pole shoe surfaces. A further essential disadvantage is that the effect of shimming subsequently cannot be varied any more and the effect also depends on the operating point of the magnet (magnitude of the magnetic flux density).